RESEARCH ARTICLE Morphology of the Jaw-Closing Musculature in the Common Wombat (Vombatus ursinus) Using Digital Dissection and Magnetic Resonance Imaging Alana C. Sharp*, Peter W. Trusler School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia * [email protected] Abstract Wombats are unique among marsupials in having one pair of upper incisors, and hypsodont molars for processing tough, abrasive vegetation. Of the three extant species, the most abundant, the common wombat (Vombatus ursinus), has had the least attention in terms of masticatory muscle morphology, and has never been thoroughly described. Using MRI and digital dissection to compliment traditional gross dissections, the major jaw adductor mus- cles, the masseter, temporalis and pterygoids, were described. The masseter and medial OPEN ACCESS pterygoid muscles are greatly enlarged compared to other marsupials. This, in combination Citation: Sharp AC, Trusler PW (2015) Morphology with the distinctive form and function of the dentition, most likely facilitates processing a of the Jaw-Closing Musculature in the Common tough, abrasive diet. The broad, flat skull and large masticatory muscles are well suited to Wombat (Vombatus ursinus) Using Digital Dissection generate a very high bite force. MRI scans allow more detail of the muscle morphology to and Magnetic Resonance Imaging. PLoS ONE 10(2): be observed and the technique of digital dissections greatly enhances the knowledge ob- e0117730. doi:10.1371/journal.pone.0117730 tained from gross dissections. Academic Editor: Kornelius Kupczik, Max Planck Institute for Evolutionary Anthropology, GERMANY Received: August 6, 2014 Accepted: January 2, 2015 Published: February 23, 2015 Introduction Copyright: © 2015 Sharp, Trusler. This is an open Many morphological characteristics reflect the grazing nature of wombats. Like other members access article distributed under the terms of the of the order Diprotodontia, wombats have only two lower incisors, but they are unique among Creative Commons Attribution License, which permits marsupials in having only one pair of upper incisors. The cheek teeth, one premolar and four unrestricted use, distribution, and reproduction in any molars on each side of the jaw, have a flat surface, and are rootless, allowing them to grow con- medium, provided the original author and source are tinuously throughout the animal’s life. When feeding, the lower jaw moves sideways and up- credited. wards so that the blades of the molars meet to cut the grass stems [1,2]. They have a bilateral Data Availability Statement: All relevant data are chewing function where only the muscles on the working side activate during the power stroke within the paper and its Supporting Information files. [3,4]. This is an efficient way of processing the course, abrasive grasses that are the wombat’s Funding: These authors have no support or funding dominant food [1]. to report. The common wombat, Vombatus ursinus, is the largest of the three species of wombats, Competing Interests: The authors have declared weighing between 23 and 38 kg, and having a body length from 850 to 1140 mm [5,6]. The that no competing interests exist. northern hairy-nosed wombat (Lasiorhinus krefftii) is about the same size as V. ursinus, but the PLOS ONE | DOI:10.1371/journal.pone.0117730 February 23, 2015 1/19 Jaw-Closing Muscles of the Common Wombat southern hairy-nosed wombat (Lasiorhinus latifrons) is slightly smaller at 22–25 kg and 850 mm in length [1]. Of these species, V. ursinus is the most abundant. However, it has had the least attention in terms of masticatory muscle morphology, and has never been thoroughly de- scribed. In addition, conclusions have been drawn for both V. ursinus and L. latifrons without any published illustrations or descriptions of the muscle anatomy for V. ursinus. While these genera are considered to be anatomically similar, they live in very different environments and have different diets [1]. Thus, any conclusions based on soft-tissue morphology and function of L. latifrons, may not apply for V. ursinus. Murray [7] and Crompton et al. [4] detailed the anatomical features of the jaw muscles for L. latifrons. However, there is little consistency in the terminology and measurements of mus- cle weights between the two studies. The designation of the three major masticatory groups, m. temporalis, m. masseter and m. pterygoideus, needs detailed reassessment and possible re- defining. Detailed studies of the muscle structures in macropodoidea, and a wide range of placental herbivores, indicates that the masticatory muscle system is more complex than those typically documented for carnivores [8–11]. The added complexity arises as a result of incomplete fascia that give rise to interdigitating muscle bands within and between mastica- tory groups. Uncertainties typically concern the masseter and temporalis and the number of components into which each muscle divides. Confusion may be compounded by the misiden- tification of complete and partial divisions and application of nomenclature that may not reflect homologies. A possible cause of this uncertainty is the invasive and destructive nature of dissections: superficial structures are removed and weighed first, while deeper structures desiccate and lose mass over time; blunt dissection between muscle layers often means that the spatial rela- tionships are difficult to define accurately; and, to access very deep and confined structures, like the pterygoid muscles, removing the zygomatic arch or coronoid process may be neces- sary. For these reasons, we employ an alternative method, “digital dissection”,tocompliment gross dissections of V. ursinus in order to obtain a greater understanding of the muscle anatomy. Techniques for imaging muscle anatomy have become considerably more varied over the last few years [12–17]. Computed tomography (CT) scanning, magnetic resonance imaging (MRI) and contrast enhanced CT imaging using Iodine Potassium Iodide (I2KI) and Phospho- tungstic acid (PTA), have enabled the visualisation of soft tissues to compliment more tradi- tional techniques such as dissection. These methods allow us to achieve greater understanding of the spatial arrangement, muscle fibre architecture and complexity of soft tissues, as well as providing additional means to measure volume, fibre direction and length, cross sectional area and muscle force. The following three-dimensional (3D) reconstruction and description of the jaw muscula- ture of V. ursinus was motivated by the lack of detailed anatomical data on this species. Digital, and gross dissections of the jaw adductor muscles were undertaken with the aims to: 1) provide a detailed anatomical description of the jaw adductor muscles that function to close the jaw; 2) gather quantitative data on relative muscle size; 3) map the area of the muscle origins and in- sertions; and, 4) compare the jaw muscle morphology of V. ursinus to that of L. latifrons to pro- vide data for further studies given the importance of wombats in comparative anatomy and palaeontology of marsupials. The 3D model of the skull and muscles was constructed using CT and MRI data to visualise the spatial arrangement of the muscles, and to measure the volume of each muscle as an estimate of absolute and relative muscle size. The 3D model is available as an interactive 3D Adobe Portable Document Format (3D PDF) file (S1 Fig), viewable on the freely available Adobe Reader [18,19]. PLOS ONE | DOI:10.1371/journal.pone.0117730 February 23, 2015 2/19 Jaw-Closing Muscles of the Common Wombat Materials and Methods Nomenclature For marsupial taxa, most authors agree on the terminology for the masseter layers as superficial and deep, with the inclusion of the zygomaticomandibularis as the deepest layer of the group [7,9,10,20]. However, there are some variations. For L. latifrons, Crompton et al. [4] described two layers of the superficial masseter (termed internal and external) and a deep masseter, but does not describe the zygomaticomandibularis. The internal superficial masseter is described as originating on the ventral surface of the zygomatic arch, and the deep masseter as originat- ing from the medial surface of the zygomatic arch. Based on their origins, these muscles could have been named the deep masseter and zygomaticomandibularis respectively, which are more commonly used terms. Other variations include multiple layers of the superficial masseter in Macropodoidea [9,11], and the addition of an intermediate masseter [11]. Here, the masseter will be referred to as having four layers (superficial, intermediate, deep and zygomaticomandi- bularis) based on their origins. Each of these muscles may also include multiple layers if partial fascia are present. There is less consistency in the literature for the temporalis muscle group. Most agree on three components, but the names designated for each vary. Crompton et al. [4] described the temporalis in L. latifrons as having a posterior layer and a deeper anterior layer. The deep layer has also been described as inferior in Phascolarctos cinereus [20] and middle in Macropus giganteus [9], but is generally referred to as deep in most other taxa [10,11]. To avoid confu- sion, we refer to this layer as ‘deep’. The fan shaped posterior layer is also referred to as medial [11], superior [20] and superficial [10]. We will use ‘posterior’, based on the posterior location of the origin within the temporal fossa. The third layer is often described as a division of the posterior temporalis (pars zygomatica by Turnbull, 1970) or the superficial lateral layer based on its origin on the zygomatic arch. Here, we describe the superficial lateral layer as separate from the posterior layer. Here, the pterygoid muscles are referred to as medial and lateral, rather than internal and external as in Turnbull [10], as this is the most common use in Vombatidae and most other marsupial families [4,7,9,11,20]. The medial pterygoid (referred to as m. pterygoideus internus by Turnbull [10]) is often described as having two parts: a deep and superficial part. Inconsis- tency in the nomenclature has made the distinction between these two parts unclear.